fix(ci): resolve flaky api-command module resolution and migrate to self-hosted runner

Root cause: Bun's test runner fails to resolve exports from directory
index.ts files during parallel test execution. The api-command/index.ts
imports 8 subcommand modules — if any fail to load during concurrent
test runs, the entire module fails with "Export not found."

Fix: extract createApiCommandHandler into handler.ts (lightweight, only
imports named-command-router). Test imports from handler.ts directly,
bypassing the heavy index.ts that loads all subcommand modules.

Also:
- Remove stale api-command.ts barrel file
- Use explicit /index import paths in source
- Migrate CI validate to self-hosted runner with clean artifact step
- Disable Bun cache (no-cache: true) on self-hosted runner

tests/unit/commands/api-command-router.test.ts

@@ -1,6 +1,6 @@
 import { beforeEach, describe, expect, it } from 'bun:test';
 
-import { createApiCommandHandler } from '../../../src/commands/api-command';
+import { createApiCommandHandler } from '../../../src/commands/api-command/handler';
 
 let calls: string[] = [];